1. Field of the Invention
The present invention relates to a dielectric resonator and a high-frequency band-pass filter for operation in a microwave or millimeter-wave band.
2. Description of the Related Art
The demand for mobile communication systems in the 900 MHz band and the near-microwave band has increased rapidly in recent years and a deficiency of usable frequencies may occur in the future. In order to develop systems adapted to multimedia communications, communication systems for transmitting still images or video images are being studied. Such communication systems must be realized as large-capacity high-speed communication systems. The use of the millimeter wave frequency bands, which are practically unused, has been taken into consideration because the bandwidth, the capacity and the communication speed of a communication channel can easily be increased in the millimeter wave band.
Use of millimeter waves to communicate within a small cell called a pico-cell has also be taken into consideration to make use of their characteristically large losses caused by absorption into the air. A mobile communication system using such a millimeter wave band pico-cell has a very small radio communication zone and therefore requires a much greater number of base stations than the systems using the near-microwave bands and other lower bands. Therefore, a smaller filter which is mass-producible at a lower cost in comparison with conventional filters is required for base stations for mobile communication in such a millimeter wave band pico-cell.
Conventionally, waveguide filters have generally been used as microwave and millimeter wave band filters. Recently, however, TE.sub.01.delta. mode dielectric filters using a TE.sub.01.delta. mode dielectric resonator have come into wide use in microwave bands in place of high-priced waveguide filters. TE.sub.01.delta. mode dielectric filters are constructed in such a manner that a plurality of cylindrical TE.sub.01.delta. mode dielectric resonators are arranged at predetermined intervals in a rectangular waveguide having a cut-off frequency higher than that of the dielectric resonators. A prototype of this kind of filter was reported by Cohn in 1967 and studies for putting it to practical use in broadcasting apparatuses were thereafter advanced. In 1975, Wakino et al. made a practical filter of this kind having high stability with respect to temperatures by using a temperature-characteristic-compensated dielectric. In general, the temperature characteristics of dielectric resonator filters are determined by the temperature characteristics of the material of the resonator. Therefore, dielectric resonator filters have the advantage of being free from the need for using an expensive metal such as Kovar (registered trademark of Westinghouse Co.) or Invar (registered trademark of Societe Creusot-Loire) to form the cavity.
A filter having waveguides as input and output means has been reported as an example of use of a TE.sub.01.delta. mode dielectric resonator in a millimeter wave band. A filter using as input and output means a nonradiative dielectric waveguide (hereinafter referred to as an "NRD guide") invented by Yoneyama et al. for the purpose of reducing losses has also been reported. These filters have small losses and good characteristics even in a millimeter wave band.
However, the resonance frequency of TE.sub.01.delta. mode dielectric resonators is determined by the size of a cylindrical dielectric member therein. The size of this cylindrical member is very small, that is, the height is 0.37 mm and the diameter is 1.6 mm at 60 GHz. Therefore, if a TE.sub.01.delta. mode dielectric resonator is manufactured with the level of working accuracy presently achievable in a mass-production process, variations of resonance frequency from unit to unit will be considerable.
When a TE.sub.01.delta. mode dielectric filter is constructed by using a plurality of TE.sub.01.delta. mode dielectric resonators, it is necessary to arrange the TE.sub.01.delta. mode dielectric resonators at predetermined intervals with high accuracy in a waveguide. In practice, it is difficult to manufacture the filter with such high accuracy. Therefore, in a case where a dielectric filter having desired filter characteristics in a millimeter wave band is manufactured, there is a need to provide each of a plurality of TE.sub.01.delta. dielectric resonators with a resonance frequency adjustment means for finely adjusting the resonance frequency of the resonator and there is also a need to provide, between each adjacent pair of the TE.sub.01.delta. dielectric resonators, a coupling adjustment means for finely adjusting the amount of coupling between the pair of TE.sub.01.delta. dielectric resonators, resulting in a considerable increase in manufacturing cost.